1. Field of the Invention
The present invention relates to an imaging lens and an imaging apparatus, and more particularly, to an imaging lens suitable as a reading lens for reading a document image and an imaging apparatus including the imaging lens.
2. Description of the Related Art
Image reading apparatuses have been known which read a document image using a lens, form the document image on an imaging device, such as a CCD (Charge Coupled Device), and convert the image information into signals. The image reading apparatus needs to read the document image information fully. Therefore, the imaging lens used for reading the document image needs to effectively correct various aberrations such as field curvature and distortion. In recent years, with an increase in the density of the imaging device, there is a demand for an imaging lens with high resolution. In particular, when a color document image is read, it is preferable that images formed in red (R), green (G), and blue (B) wavelength bands be substantially aligned with each other on the imaging surface of the imaging device and the contrasts of the images be equal to each other. Specifically, it is possible to obtain a high-contrast image on the imaging surface in a wide wavelength range by reducing the deviation of spherical aberration, longitudinal chromatic aberration, or lateral chromatic aberration in each of the R, G, and B colors and aligning the imaging positions of the images in the range from the center to the periphery of the images (aligning the image surfaces).
A line sensor has generally been used as the imaging device of the image reading apparatus. In general, there are a number of methods by which a line sensor scans and reads the document image. It is necessary to increase the resolution of the imaging device in order to read a document image with high resolution and it is easy for a line sensor with high selectivity to the size of the imaging element to respond to increasing resolution.
An image reading apparatus has been proposed which uses an area sensor instead of the line sensor. In recent years, the definition of an imaging device in the area sensor has increased and an image sensor with a pixel pitch of about 1 μm has been developed. For example, sensors with a small size of ⅓ inch and 500 megapixels to 800 megapixels are on the market. The use of the high-definition sensor makes it possible to read a document image with high resolution, similar to the line sensor with a wide reading width. In addition, the use of the area sensor makes it possible to read the document image all at once without scanning the document image. Therefore, it is possible to omit a scanning mechanism and allow a reduction in the size of the apparatus.
Since there is a strong demand for an image reading apparatus with a small size, it is preferable to reduce the size of the imaging lens. JP-A-2008-275783 and JP-A-2009-53411 disclose an image reading lens with four lenses and an image reading lens with three lenses which can be used for the above-mentioned image reading apparatus. JP-A-2005-18041 and JP-A-2007-122007 disclose an imaging lens with four lenses. JP-A-2005-181596 and JP-A-2007-133324 disclose an imaging lens with three lenses.
As described above, it is preferable that the imaging lens for an image reading apparatus effectively correct various aberrations and have a high optical performance capable of responding to a high-definition imaging device with a large number of pixels. In order to reduce the size of the image reading apparatus, it is necessary to reduce the focal length of the imaging lens, thereby decreasing a conjugate length (substantially, the distance from the document image to the imaging surface of the imaging device), as well as reducing the size of the imaging lens.
When the line sensor is used in the image reading apparatus, it is possible to obtain high resolution. However, in this case, since the size of the imaging device increases, it is difficult to reduce the imaging magnification of the optical system (it is difficult to increase a reduction ratio). Therefore, it is difficult to reduce the focal length of the imaging lens. When the conjugate length is reduced in this kind of apparatus, an increase in the angle of view of the optical system is noticeable. Therefore, in the imaging lens according to the related art, various aberrations are insufficiently corrected and it is difficult to read the document image with high resolution.
When the area sensor is used in the image reading apparatus, the imaging magnification is reduced (the reduction ratio increases) and it is possible to reduce the focal length. Therefore, it is relatively easy to reduce the conjugate length and it is possible to reduce the size of the apparatus. However, when a large document image is read all at once by the area sensor, the diagonal of the document image is an imaging range. Therefore, when the angle of view of the imaging lens is insufficiently wide, the conjugate length increases, which makes it difficult to reduce the size of the apparatus.
The lens systems disclosed in JP-A-2008-275783 and JP-A-2009-53411 have a large F-number of 5.2 to 6 and it is difficult for the lens system to sufficiently respond to high-definition pixels. In addition, these lens systems are not miniaturized. Therefore, when the lens system is provided in the imaging apparatus, a large imaging device is required and it is difficult to reduce the size of the apparatus. The lens systems disclosed in JP-A-2005-18041 and JP-A-2007-122007 are manufactured for portable terminals. Therefore, the lens systems have a small size and a sufficiently small F-number of 2.4 to 3.0 and can respond to high resolution. However, the lens systems have a total angle of view of about 75° at most. Therefore, when a large document image is read all at once by the area sensor, the angle of view is insufficiently wide and it is difficult to reduce the size of the apparatus. The lens systems disclosed in JP-A-2005-181596 and JP-A-2007-133324 have a small size and an angle of view wider than the lens systems disclosed in JP-A-2005-18041 and JP-A-2007-122007. However, the lens system is insufficient to correct chromatic aberration and it is difficult for the lens system to obtain high resolution.